1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a rigid magnetic recording disk having improved magnetic properties, and a method for its production.
2. Discussion of the Background
In recent years, the application range of magnetic recording apparatus, such as magnetic disk drives and flexible disk drives, has been remarkably expanded and their importance has increased. At the same time, with respect to magnetic recording media to be used for such apparatus, remarkable improvements in the recording density are being made.
With respect to such magnetic recording media, it is necessary to increase the recording density further. For this purpose, the coercivity must be increased and the signal to noise ratio (SN ratio) of the magnetic recording layer must be increased.
It is generally known that the following relationships exist between the properties of a magnetic recording medium and the linear recording density, the output and the SN ratio:
(Linear recording density).varies.(Hc/Br-t) PA1 (Output).varies.(Br.t.Hc) PA1 (SN ratio).varies.(Hc/Br.t)
where Hc represents the coercivity, Br represents the residual magnetic flux density, and t represents the thickness of the magnetic layer. Further, the symbol .varies. indicates that the property on the left side is proportional to the value on the right side.
Accordingly, in designing a magnetic recording medium having a high recording density-, it is necessary to increase the coercivity, while maintaining Br.t without reducing the required output.
In recent years, with a view to attaining the high recording density, thin metal layer-type magnetic recording media have been used instead of coating-type magnetic recording media where the magnetic layer is formed by coating a magnetic coating material comprising a magnetic particle and a binder resin.
In such thin metal layer-type magnetic recording media, the magnetic layer is formed by a method such as electroless plating, electroplating, sputtering or vapor deposition. As the composition for such magnetic layer, a Co(cobalt)-P(phosphorus) alloy, a Co-Ni(nickel)-P alloy, a Co-Ni-Cr(chromium) alloy, a Co-Ni-Pt(platinum) alloy or a Co-Cr-Ta(tantalum) alloy has been practically employed.
Recently, it has been reported that in a layer-forming method by sputtering, a high coercivity is obtainable by applying a negative bias voltage to the substrate during the formation of the magnetic layer (The 35th Spring Meeting the Japan Society of Applied Physics and Related Societies in 1988, Reference Materials 29a-C-9 and -10, and Meeting of Technical Group on Component Parts and Materials of the Institute of Electronics, Information and Communication Engineers, Reference Material CPM88-92, Japanese Unexamined Patent Publication No. 34,324/1982).
It is known that high coercivity is obtainable by adding Pt to a Co-Cr type magnetic layer (Japanese Unexamined Patent Publication No. 88,806/1984) or to a Co-Cr-Ta type magnetic layer (U.S. Pat. No. 5,024,903). However, it is difficult to achieve both high performance and low price, using expensive materials such as platinum as a component of the magnetic layer.
In the meantime, thin-film magnetic recording media having a Co-Ni-B(boron)-transition metal type magnetic layer or a Co-Ni-Cr-B type magnetic layer (Japanese Unexamined Patent Publication No. 18,607/1988, No. 49,021/1991) have been proposed. However, the above-mentioned thin-film magnetic recording media are still insufficient in coercivity.